This invention relates to an improved horizontal sandmill or media mill. Sandmilling is a proven, practical, continuous, high production method of dispersing and milling particles in a liquid to produce smooth, uniform, finely dispersed products. One use of sandmilling is to mill pigments in paints. The process is also applicable to the production of a wide variety of inks, dyestuffs, paper coatings, chemicals, magnetic tape coatings, insecticides, and other materials in which milling to a high degree of fineness/dispersion is required.
In a typical sandmilling process, the material or slurry to be processed is introduced at the bottom of a vertically oriented processing chamber or vessel and pumped upwards through a grinding media. The grinding media in years past had usually been sand, but more commonly a small diameter manufactured product of steel or other material is used as the grinding media. Rotors within the vessel agitate the slurry and the grinding media to insure proper particle reduction and dispersion of the slurry.
Typically, the processed liquid exits the vessel at the upper end of the vessel. Since the liquid product may contain suspended particles of the grinding media, generally a screen is placed at the vessel's outlet to retain the media in the mill.
While media mills having a vertical orientation, are in widespread use, horizontal mills are desired in some situations. Because of height limitations, the shaft of the motor which drives the rotor generally is parallel to the rotor itself, and requires gears or belts to drive the rotor. Also, with tail mills the motor and transmission are not readily accessible. Further, since the media gravitates toward the bottom of the vessel, wear on the rotor discs is uneven, thus requiring replacement before all are worn.
A horizontally oriented vessel requires more floor space but has some advantages. With horizontal orientation, height problems are reduced, and all of the apparatus is readily accessible. Also, the rotor wear tends to be more even in horizontal mills, since the media is distributed fairly evenly throughout the vessel. However, replacing the rotor in prior horizontal mills requires a lengthy disassembly process, as does replacing the media.
Prior vertical mills have taken advantage of the media's gravitational tendency by utilizing a dump valve at the lower end or by fastening an end plate to the open bottom end of the vessel by means of a quick disconnect ring, as disclosed in U.S. Pat. No. 4,140,283, owned by the assignee of the present invention. Removing the end plate or opening the valve allows the media to quickly drain out the bottom of the vessel for cleaning or replacement.
Since the media lays along the bottom of a horizontal vessel for the entire length of the vessel, removal of the media or access to the screen or rotor has been troublesome. Prior horizontal mills are typically formed of sections which are bolted together, and these sections must be disconnected. To access all of the media, usually results in media spilling out along the entire length of the vessel. To aid in this process, a large pan or tray may be utilized, with the result that the media must again be transferred to another container. The entire operation is messy and time consuming.
Regardless of the orientation of the vessel, prior mills have also been unsatisfactory in other respects. The screen placed at the vessel's outlet is generally a welded steel screen which becomes worn with use due to the constant contact with the grinding media, and must be frequently replaced. Replacement is a very time-consuming operation. In the event that a flammable product is being milled, the controls for the electric motor used to drive the rotor must have explosion-proof switch boxes, adding to the overall expense of the mill.
The cost and complexity of the mill may also be increased by the need for a pump to circulate coolant through a cooling jacket surrounding the vessel, to remove the heat generated by the friction of the milling process. Also, a separate pump is required to pump product through the vessel. Each pump usually means a corresponding electric motor, and therefore explosionproof switch boxes as well.
The mill rotor is usually formed of a plurality of separate discs bolted or keyed to a shaft. This can introduce balance problems, if adequate tolerances and manufacturing accuracy are not maintained. It also requires considerable assembly labor.
Thus, a need exists for a simplified media mill which is easily cleaned, has a long lasting and easily replaceable rotor and screen, and requires fewer explosion-proof switch boxes, pumps, and electric motors.